The study, published in the September issue of Journal of Health Economics found that friends' weight is correlated with an adolescent's own weight even after considering demographics, smoking status, birth weight, and household characteristics such as parental obesity.
"Our results may help explain the dramatic rise in obesity among adolescents in the past few decades," said Justin Trogdon, Ph.D. a health economist at RTI and the paper's lead author.
"Peers can influence all of the significant weight-related choices for teens, including eating patterns, diets and physical activity. Peers also affect teens' perceptions of an acceptable weight," the researcher added.
The researchers found the peer effect on weight was strongest among females, and among adolescents who were at risk of becoming overweight.
The study also showed that teens with obese parents were more likely to be overweight themselves.
The study used data from the National Longitudinal Study of Adolescent Health, a study that RTI helped conduct, that surveyed youths in grades 7 through 12.
The researchers looked at students from 16 schools included in the data and defined peer groups based on friendships and grade level.
"Research has also indicated that peers may help adolescents to lose weight," Trogdon said.
"Better understanding peer influence on weight will help improve policies and prevention efforts aimed at reducing adolescent weight," the expert added.